The present invention relates to thermosetting resin coating powder compositions having enhanced electrostatic chargeability. The coatings are applied using conventional electrostatic equipment and then fused and cured. The coating powder compositions of the invention include inert nitrogen-containing compounds that function as electrostatic charge enhancing agents. By inert, it is meant that these compounds are not chemically reactive with other compounds of the coating powder under the conditions the compounds are normally exposed, i.e., during powder manufacturing and coating. Melamine, urea, dicyandiamide, and benzoguanamine and derivatives thereof are inert nitrogen-containing compounds that enhance chargeability. The use of these coating powders eliminates or minimizes the need for the surface preparation of non-conductive substrates prior to electrostatic application and serves to improve coating thickness, uniformity, and quality.
Coating powders are extremely desirable for coating substrates because such powders are virtually free of the fugitive organic solvents that are conventionally used in liquid paint systems. Powder coatings, thus, emit few, if any, volatile materials to the environment when heat or radiation cured. Problems of air pollution and health dangers to workers employed in painting operations are thereby reduced.
Solventless fusion coating processes have been developed to apply coating powders on substrates in which dry, finely divided, free flowing heat fusible powders are deposited on the substrates and then fused and cured with heating to form continuous, protective or decorative films. Examples of such coating processes include electrostatic spray and fluidized bed techniques; with the electrostatic spray technique being more frequently used in industry.
Although powder coatings have many benefits, it is difficult to coat electrically non-conducting substrates such as plastic, wood, and wood composites. The two main challenges are the need for low curing temperatures and the need for enhancement of the electrostatic attraction of coating powder to non-conductive substrates. The past several years, new technologies have been introduced into the market for providing low temperature cure powder coatings. U.S. Pat. Nos. 5,714,206 and 6,077,610 involve two-component coating powders that can be cured at the low temperatures required for wood and a variety of other metallic and non-metallic substrates. This result is achieved by mixing resin and curing agent components following extrusion in the dry form. The procedure eliminates heating, and thus reaction, of the two components together until they are deposited on the substrate. U.S. Pat. No. 5,721,052 discloses epoxy resins cured with imidazole adducts to obtain low temperature cure powder coatings, especially black textured coatings. U.S. Pat. No. 5,824,373 discloses UV curable powder coating to further reduce cure temperatures through the use of UV radiation curing. All of the above patents address the low cure temperature aspect of the powder coatings for non-conductive substrates and thus address the problem of substrate degradation at normally used higher powder coating cure temperatures. A further challenge remains in the enhancement of the attraction of coating powders to non-conductive substrates prior to curing.
Powder attraction techniques, such as those discussed below, are known in the art.
U.S. Pat. No. 5,731,043 discloses producing coating powders of controlled particle size distribution to improve tribo-charging characteristics for wood and metal substrates. Several other techniques have been developed to impart sufficient electrical conductivity to the substrate so that the substrate can be electrostatically powder coated. A conductive material, such as graphite, can be added to the substrate to improve substrate conductivity. However, this technique has the disadvantage that modification of the character of the substrate is required.
European Patent Publication No. 0933140A1 discloses a method where the substrate can be preheated to cause the powder particles to partially cure and adhere when the particles initially contact the heated surface. Preheating wood substrates also assists in driving moisture to the surface and thereby assisting the electrostatic application. Due to the varying nature of the wood substrates, controlling the moisture content is very difficult. Should the moisture content be too low, even preheating the part will not sufficiently improve electrostatic application. Excessive preheating to improve conductivity can also be detrimental to the substrate. Moreover, preheating parts that have sharp edges permits moisture to escape more rapidly from such edges than from thicker areas, thereby rendering powder application very difficult.
A typical solution to the electrostatic application problem is to apply a conductive primer to the substrate prior to powder application. This approach is illustrated in U.S. Pat. No. 5,344,672 where an electrically conductive primer, typically containing metallic or graphite particles, is coated onto the surface of the substrate. Although such approach is operable, it interposes an electrically conductive coating between the substrate and the cured powder coating and thus requires an additional process step that is not required by the present invention. Electrically conductive coatings can interfere with some intended uses of the finished part, which otherwise would not exhibit electrical conductivity. The present invention does not require such conductive primer.
Still another approach involves the application of an antistatic material to the substrate prior to coating powder application. Patent Publication No. WO98/58748 and U.S. Pat. No. 4,686,108 disclose that the use of conductive polymeric coatings having a charge density of greater than 2, after being applied to nonconductive substrate, permits subsequent overcoating of the substrate by electrostatic spraying. The antistatic coating, typically on the order of a few micrometers thick or less, provides sufficient electrical conductivity to the surface to permit electrostatic powder coating. The surface conductivity of the antistatic-coating substrate is about 1012 ohms per square or more, and may be adjusted by heat treatments. Such high resistivity does not result in unacceptable electromagnetic wave attenuation for most end use applications. A limitation of this approach is that multiple steps are involved in finishing operations. The present invention does not require such antistatic materials.
Antistatic agents most commonly are amines, such as tertiary amines, or ammonium compounds, such as quaternary ammonium compounds. These agents cannot be incorporated into coating powder formulations because such agents are commonly used at very low levels (less than 0.5 part per 100 parts of resin) as catalysts in many coating powder formulations. This approach is limited with respect to the amount of material that can be incorporated in the formulation because ammonium compounds also function as cure catalysts for most coating powder formulations.
Finally, U.S. Pat. No. 6,113,980 discloses improving chargeability of powder coatings using electron donor compounds for coating metal substrates by tribocharging the coating powders. Certain hindered amines are disclosed as such compounds. The implementation of this approach has been hindered by the lack of commercial availability of the charge control agents as well as the cost of these materials. The chemical structure of these hindered amine compounds can impart performance characteristics to the coating powders that may not be desirable, at least in some formulations. The patent requires that the electron donor compounds comprise at least two different compounds containing a residue of the following formula: 
On the other hand, the present invention utilizes electrostatic charge enhancement compounds for powder coating non-conductive substrates that are quite structurally distinct from that of said U.S. Pat. No. 6,113,980. Moreover, the present invention requires only a single ingredient and is thus less cumbersome.
There remains a need for further improved technique to enhance the electrostatic powder coating of electrically non-conducting and electrically conducting substrates. Such technique would find widespread application in the coating of, for example, wood, wood composite materials, ceramics, glass, plastics, and the like as well as metal. It is believed that such enhancement not only fulfills a long-standing need in the art but also provides additional advantages.
The coating powders of the invention include as a component an inert nitrogen-containing compound such as melamine, urea, benzoguanamine, dicyandiamine, and derivatives of such compounds. The term xe2x80x9cinertxe2x80x9d means that the compounds do not react with other components of the coating powder during manufacturing steps of the powder, such as extrusion, or during coating temperatures when the powder is applied to a substrate. Thus, the essentially unreacted compound is free to perform its function as an electrostatic charge enhancement agent during the coating process. The balance of the powder is a thermosetting resin polymer that may contain other ingredients that are typically included in coating powders.
In one advantageous aspect, the process of the invention involves coating non-conductive substrates by providing the above described coating powder, electrostatically charging said powder, applying the charged powder to a non-conductive substrate, and then curing the applied powder by use of heat or radiation. Curing is conducted at temperatures below about 300xc2x0 F. for heat sensitive substrates such as wood and plastic to avoid damage to such substrates. Curing temperatures greater than about 300xc2x0 may be used for other non-conductive substrates capable of withstanding higher curing temperatures such as glass, ceramics, etc. Because of the enhancement of chargeability of the coating powders, conductive substrates, such as metals, may also be coated. The prsent invention is especially useful where thin or complex cross-sections are being coated because coating uniformity is enhanced by use of the coating powders of the present invention. The coated article of the present invention results from the above-described process.